Erdos, Paul, AYR dohsh, powl (1913-1996), was a Hungarian mathematician who ranked as one of the greatest mathematicians of the 1900's. He was well known for his work in number theory, a branch of mathematics concerned with the properties of whole numbers, also called natural numbers. Erdos was also a pioneer in a branch of mathematics called combinatorics. Combinatorics deals with permutations, different arrangements of the numbers or other members in a group, and with combinations, groups that include the same members regardless of how they are arranged. See Permutations and combinations.